Conventional X-ray sources typically use a thermionic emission mechanism to produce electrons that impinge on a metallic target from which x-rays are emitted through bremsstrahlung processes. Thermionic emission entails the emission of electrons by heated filaments. Thermal inertia limits the time resolution of thermionic systems to microseconds, while spatial resolution of the electron emitter is governed by the dimension of the filament.
X-ray sources may also be based on field-emission cathodes, offering advantages in both spatial and temporal resolution when compared with thermionic sources. Because field emission of electrons is produced by a high electric field, no heating is necessary, whence such electron emitters are commonly referred to as cold cathodes. The electron beams emitted by such devices may have low divergence and thus provide ease of focusing. Moreover, the virtually instantaneous response of the source offers time gating capabilities comparable with the time resolution of the control circuit, and may be as fast as nanoseconds, using current technology.
Zhang et al., A Multi-beam X-ray Imaging System Based on Carbon Nanotube Field Emitters, in Medical Imaging 2006, (Proceedings of SPIE, Vol. 6142, Mar. 2, 2006), reported the fabrication, by Xintek, Inc. of Research Triangle Park, N.C., of a linear array of 5 X-ray sources, each with a focal spot between 200 and 300 μm, based on the use of carbon nanotube (CNT) electrodes. Electron currents in the range of 0.1-1 mA were reported at an accelerating voltage of 40-60 kVp. The lifetime of the cold cathode was estimated to exceed 2000 hours. For an accelerating voltage of 200 kV, a beam current of 13 mA has been measured. The aforesaid Zhang et al. paper is incorporated herein by reference. Devices with 1000 pixels per meter and pulse repetition rates on 10 MHz can be envisioned with technology within the current state of the art.
The use of CNT cold cathodes in the context of an X-ray source is also described by Cheng et al., Dynamic radiography using a carbon-nanotube-based field-emission X-ray source, 75 Rev. Sci. Instruments, p. 3264 (2004), while the use of CNT cold cathode source arrays in a scanning context is described by Zhang et al., Stationary scanning x-ray source based on carbon nanotube field emitters, 86 Appl. Phys. Lett., p. 184104 (2005), both of which articles are incorporated herein by reference.
Moreover, the use of CNT cold cathode source arrays in tomography is discussed by Zhang et al., A nanotube-based field emission x-ray source for microcomputed tomography, 76 Rev. Sci. Instruments, p. 94301 (2005), which is also incorporated herein by reference.
The footprint and throughput of personnel inspection equipment at airports have become increasingly important considerations as security checkpoints become more congested. Typically the critical dimension is that which is perpendicular to the flow of traffic, and currently deployed metal detectors are used as a standard of comparison. At US airports, the desired throughput is consistent with support of two adjacent baggage systems. This translates to a throughput of approximately 400 people per hour.
While backscatter x-ray scanning of personnel is often considered the most effective method for detection of concealed contraband goods or potential threats, current implementations of this technology may provide inadequate throughput for certain applications, and, moreover, may fail to meet size constraints imposed by certain venues such as airports. The present invention advantageously provides a solution to these shortcomings.
Current solutions that attempt to improve throughput over single-sided backscatter screening rely on several single point focus x-ray sources. Because the x-ray beam of these sources is collimated into a fan beam shaped line of x-rays, the angle of incidence on the person being screened can cause distortion in the image data. Even though the distortion can be compensated for in software, the data still derive from x-rays that vary in incident angle. At steep angles this can result in x-ray incident shadows (from body parts such as shoulders) and thus, in turn, cause concealed contraband to be missed. If several conventional x-ray sources are used to mitigate this effect, the equipment size becomes large and the equipment cost becomes unattractive.